Note: Descriptions are shown in the official language in which they were submitted.
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Process for biocking underground formations
The present invention provides a process for blocking underground formations
during
the production of fossil oil and/or gas.
In the production and extraction of liquid or else gaseous hydrocarbons from
underground formations, the greatly varying porosities of the underground rock
formations, especially in the presence of fissured rock formations, which are
referred to
as "fractured reservoirs", can cause problems.
Frequently, in the course of so-called secondary or tertiary oil production
measures,
water, steam or aqueous polymer solutions are pumped into the formation, in
order
thus to force the hydrocarbons out of the formations and the fissures present
therein. In
the case of these measures, differences in the porosities of the formation
lead to the
effect that the particular flooding medium preferentially flows through the
permeable
channels, such that zones of lower porosity are flooded only incompletely, if
at all. As a
result, exclusively those hydrocarbons which are within the more permeable
rock
regions then become accessible and extractable.
In order to prevent this, attempts are therefore made to block the more
permeable
channels selectively, for which, for example, water-swellable polymer
particles which
are also referred to as superabsorbents or superabsorbent polymers (SAPs) are
used
(cf. US 4,182,417).
Superabsorbent polymers are crosslinked, high molecular weight, either anionic
or
cationic polyelectrolytes which are obtainable by free-radical polymerization
of suitable
ethylenically unsaturated vinyl compounds and subsequent measures for drying
the
resulting copolymers. On contact with water or aqueous systems, a hydrogel
forms with
swelling and water absorption, in which case several times the weight of the
powdery
copolymer can be absorbed. Hydrogels are understood to mean water-containing
gels
based on hydrophilic but crosslinked water-insoluble polymers which are
present in the
form of three-dimensional networks.
Superabsorbent polymers are thus generally crosslinked polyelectrolytes, for
example
consisting of partly neutralized polyacrylic acid. They are described in
detail in the book
"Modern Superabsorbent Polymer Technology" (F.L. Buchholz and A.T. Graham,
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2
Wiley-VCH, 1998). In addition, more recent patent literature includes a
multitude of
patents which are concemed with superabsorbent polymers.
Reference is made here by way of example to the following documents:
US 5,837,789 describes a crosslinked polymer which is used for absorption of
aqueous
liquids. This polymer is formed from partly neutralized monomers with
monoethylenically unsaturated acid groups and optionally further monomers
which are
copolymerized with the first component groups. A process for preparing these
polymers
is also described, wherein the particular starting components are first
polymerized to a
hydrogel with the aid of solution or suspension polymerization. The polymer
thus
obtained can subsequently be crosslinked on its surface, which should
preferably be
done at elevated temperatures.
Gel particles with superabsorbent properties, which are composed of several
components, are described in US 6,603,056 B2. The gel particles comprise at
least
one resin which is capable of absorbing acidic, aqueous solutions, and at
least one
resin which can absorb basic, aqueous solutions. Each particle also comprises
at least
one microdomain of the acidic resin, which is in direct contact with a
microdomain of
the basic resin. The superabsorbent polymer thus obtained is notable for a
defined
conductivity in salt solutions, and also for a defined absorption capacity
under pressure
conditions.
The emphasis of EP 1 393 757 B1 is on absorbent cores for nappies with reduced
thickness. The absorbent cores for capturing body fluids comprise particles
which are
capable of forming superabsorbent cores. Some of the particles are provided
with
surface crosslinking in order to impart an individual stability to the
particles, so as to
give rise to a defined salt flow conductivity. The surface layer is bonded
essentially
noncovalently to the particles and it contains a partly hydrolysable, cationic
polymer
which is hydrolysed within the range from 40 to 80%. This layer has to be
applied to the
particles in an amount of less than 10% by weight. The partly hydrolysed
polymer is
preferably a variant based on N-vinylalkylamide or N-vinylalkylimide, and
especially on
N-vinylformamide.
Superabsorbent hydrogels coated with crosslinked polyamines are also described
in
Intemational Patent Application WO 03/0436701 Al. The shell comprises cationic
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3
polymers which have been crosslinked by an addition reaction. The hydrogel-
forming
polymer thus obtainable has a residual water content of less than 10% by
weight.
A water-absorbing polymer structure surface-treated with polycations is
described in
German Offenlegungsschrift DE 10 2005 018 922 Al. This polymer structure,
which
has also been contacted with at least one anion, has an absorption under a
pressure of
50 g/mz of at least 16 g/g.
Superabsorbent polymers coated with a polyamine are the subject matter of
WO 2006/082188 Al. Such superabsorbent polymer particles are based on a
polymer
with a pH of > 6. The hygiene articles which have also been described in this
connection exhibit a fast absorption rate with respect to body fluids.
Superabsorbent polymer particles coated with polyamines are also disclosed by
WO 2006/082189 Al. A typical polyamine compound mentioned here is
polyammonium carbonate. In this case too, the fast absorption of body fluids
by the
particles is at the forefront.
A typical preparation process for polymers and copolymers of water-soluble
monomers
and especially of acrylic acid and methacrylic acid is disclosed in US patent
4,857,610.
Aqueous solutions of the particular monomers which contain polymerizable
double
bonds are subjected at temperatures between -10 and 120 C to a polymerization
reaction so as to give rise to a polymer layer of thickness at least one
centimetre.
These polymers obtainable in this way also possess fast superabsorbent
properties.
Both the superabsorbent polymers described in Buchholz and those described in
later
patent applications are so-called "rapid" products, i.e. they attain their
full water
absorption capacity within a few minutes. In the case of use in hygiene
articles in
particular, it is necessary that liquids are absorbed as rapidly as possible
in order to
prevent them from running out of the hygiene article.
For the application for selective blocking of underground formations in the
production of
oil and/or gas, this rapid swelling, however, presents problems, since the SAP
first has
to be introduced over relatively long distances at its site of action, the
underground
formation.
International Patent Application W096/02608 Al solves this problem by
dispersing the
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4
sulphonated superabsorbent based on 2-acrylamido-2-methylpropanesulphonic acid
(AMPS) in hydrocarbons. This measure ensures that the superabsorbent swells
only
when it comes into contact with water at its site of use.
US 2004/0168798 also describes the use of superabsorbents in the field of use
already
addressed. According to this publication, crosslinked polyacrylamides are used
as
superabsorbents. Here, too, the superabsorbent is transported for use in the
formation
by using a nonaqueous medium as a carrier fluid. A solution of CaC12 is also
said to be
suitable as a carrier medium. Depending on the concentration of the salt
solution, the
swelling of the superabsorbent can be retarded: this means that the higher the
concentration of CaC12 in the carrier liquid, the longer the complete swelling
of a
superabsorbent takes. However, a disadvantage of this proposed process is that
this
delays only the end point of the swelling, but not the commencement of
swelling.
Additionally, in US 5,701,955 there is discussed a method for the reduction of
permeability over water comprising the insertion of a dispersion of water
swellable
polymer particles with a particle size < 10 pm in a non-aqueous solvent.
The use of very small particle sizes is also disclosed by US 5,735,349. The
particles of
the superabsorbent show sizes of from 0.05 to 1 pm and are also used as a
dispersion
in a non-aqueous solvent.
In contrast, US 2003/149212 discloses a superabsorbent with a retarded
swelling
containing besides usual used stable crosslinkers additionally instable cross
linkers.
These stable crosslinkers may hydrolize in the formation at higher
temperatures and
thereby allow the swelling of the particles. The disclosure of this document
is limited to
particle sizes of from 0.05 to 10 pm. This limitation restricts significantly
possible
application forms of such systems.
A similar system is discussed by WO 2007/126318. Also in this document there
is
disclosed a combination of stable and instable crosslinkers. An important
aspect of this
system is the production of the particles in an oil-in-oil emulsion.
Another alternative for the retardation of the swelling of superabsorbents is
disclosed
by US 2008/108524. In this case the superabsorbent is coated with a shell of a
water
soluble polymer. This shell in a first step hinders the swelling of the
superabsorbent
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and in an additional step is disintegrating in such that a swelling is
possible.
One approach to a solution can thus be considered in each case to be that of
using
suitable superabsorbents. Superabsorbents with retarded swelling are described
in the
5 priority application DE 10 2008 030 712.2 which was yet to be published at
the priority
date of the present application.
Due to the shortcomings in connection with the production of oil and/or gas
which have
been outlined at the start, the present invention has for its object to
provide a novel
process for blocking underground formations in the production of fossil oil
and/or gas, a
first step involving introducing water-absorbing particles into liquid-bearing
and porous
rock formations, said particles being water-swellable, crosslinked and water-
insoluble
polymers and these particles in the water-bearing rock formation finally
preventing
liquid flow through the rock layers by water absorption. The novel process
should be
very easy to perform and not need any carrier liquids. At the same time, it
must be
ensured that the permeable channels are blocked sufficiently selectively that
the
flooding medium selected reaches all rock areas from which the hydrocarbons
are to
be extracted.
This object is achieved with the aid of the process according to the
invention, in which
the absorbing particles comprise a superabsorbent polymer (SAP) with anionic
and/or
cationic properties and a retarded swelling action.
It has been found that, surprisingly, this process - which preferably does not
need any
nonaqueous carrier liquid - and in particular the use of the specific
absorbing particles
does not just completely satisfy the objective, but the rheology jump already
known,
resulting from the absorption of liquid into the superabsorbent polymers used
in
accordance with the invention, is also achieved in underground formations.
This was
unforeseeable because the fossil reservoirs are in an environment which is
characterized by high temperatures and high pressures. An additional factor is
the flow
behaviour in the gaps and fissures of the rock formations, which constitutes a
considerable difference from the fields of use in construction chemistry in
which,
according to the unpublished priority application DE 10 2008 030 712.2, the
superabsorbent polymers described have found use to date.
It has been found, advantageously, that superabsorbents which have been
prepared by
polymerizing ethylenically unsaturated vinyl compounds are particularly
suitable.
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6
It was also unexpected that the particular superabsorbents are useable in
saline
formation waters in the process according to the invention. The present
invention
therefore claims a corresponding process variant. This was all the more
astonishing
since it is known from the prior art that salt solutions retard the swelling
of the
superabsorbent, and it was therefore expected in the present case that, when
the
superabsorbents are used in saline formation waters, the controlled adjustment
of
retarded swelling is impossible.
The present invention also encompasses the possibility that the swelling of
the
superabsorbent begins no earlier than after five minutes and the
superabsorbent has
been prepared with the aid of process variants which are selected from four
alternatives. These alternatives are a) polymerizing the monomer components in
the
presence of a combination consisting of at least one crosslinker non-
hydrolysable
under the conditions of the application and at least one crosslinker having a
hydrolysable carbonic acid ester function under the conditions of the
application;
b) polymerizing at least one permanently anionic monomer and at least one
cationic
monomer that can release its cationic charge under the conditions of the
application by
a hydrolysis of the ester function and/or a deprotonation ; c) coating a core
polymer
component with at least one further polyelectrolyte as a shell polymer; d)
polymerizing
of at least one, not hydrosable under the contions of the application, monomer
with at
least one monomer, having a hydrolysable carbonic acid ester function under
the
conditions of the application, in the presence of at least one crosslinker.
The process
variants a) to d) mentioned can be combined with one another in any number.
Owing to the specific field of use, advantageous superabsorbents are in
particular
those which have a high water absorption capacity even at moderate to higher
salt
concentrations, especially high calcium ion concentrations. The expression
"retarded
swelling action" shall be understood in accordance with the invention to mean
the fact
that the swelling, i.e. the liquid absorption, of the superabsorbent begins no
earlier than
after 5 minutes. In accordance with the invention, "retarded" means that, in
particular,
the predominant portion of the swelling of the superabsorbent polymer occurs
only after
more than 10 minutes, preferably after more than 15 min and more preferably
only after
more than 30 min. Under quantitative aspects that means that the timely
retarded
superabsorbent during its application in a carrier liquid and during its
introduction into
the formation, after 5 minutes shows less than 5 % and after 10 minutes less
than 10 %
of its maximum in swelling. In connection with hygiene articles, retardation
in the region
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7
of a few seconds has already been known for a long time, in order that, for
example,
the liquid is first distributed within the nappy before it is absorbed, in
order to be able to
exploit the entire amount of superabsorbent in the nappy and to need a smaller
amount
of nonwoven material. In the present case of the invention, however,
retardation is
understood to mean longer periods of more than 5 minutes and especially more
than
minutes.
As already discussed, the superabsorbent polymers retarded in accordance with
the
invention can preferably be provided in four embodiments:
Polymerization involving a
a) combination of a first crosslinker, not hydrolysable under the conditions
of its
application, and of a second crosslinker, showing a hydrolysable carbonic acid
ester function under the conditions of its application; or/and
b) polymerization of a permanently anionic monomer and a cationic monomer
suitable for the release of its cationic charge by an ester hydrolysis and/or
a
deprotonation under the conditions of its application; or/and
c) coating of a superabsorbent polymer as a core with a further
polyelectrolyte as
a shell, the core polymer comprising non-hydrolysable crosslinkers under the
conditions of its application; or/and
d) polymerization of at least one non-hydrolysable monomer with at least one
monomer, showing a hydrolysable carbonic acid ester function under the
conditions of its applications, in the presence of at least one crosslinker.
Each of embodiments a), b), c) or d) can be used alone. This is referred to
hereinafter
as "pure embodiment". However, it is also possible to combine the inventive
embodiments with one another. For instance, a polymer according to embodiment
a)
can be coated with a further polyelectrolyte in an additional process step
according to
embodiment c), in order to establish the retardation even more exactly. This
is referred
to hereinafter as "mixed embodiments". What is common to all embodiments,
whether
pure or mixed, is that the properties of the resulting retarded superabsorbent
polymer
correspond to the profile of requirements. In each of the embodiments, the
introduction
of the inventive retarded superabsorbent polymer, for example into a
construction
material mixture, results in a chemical reaction which leads to an enhancement
of the
absorption. After the reaction, the maximum absorption is attained, which is
referred to
hereinafter as final absorption.
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8
After the following features which cover all variants, first the pure
embodiments will be
described, before mixed embodiments are finally discussed.
The inventive SAPs are notable especially in that the particular monomer units
have
been used in the form of free acids, in the form of salts or in a mixed form
thereof.
Irrespective of the process variant used in each case to prepare the
superabsorbent, it
has been found to be advantageous when the acid constituents have been
neutralized
after the polymerization. This is advantageously done with the aid of sodium
hydroxide,
potassium hydroxide, calcium hydroxide, magnesium hydroxide, sodium carbonate,
potassium carbonate, calcium carbonate, magnesium carbonate, ammonia, a
primary,
secondary or tertiary C,-2o-alkylamine, C,-2o-alkanolamine, Cs-a-
cycloalkylamine and/or
C6-14-arylamine, where the amines may have branched and/or unbranched alkyl
groups
having 1 to 8 carbon atoms. Of course, all mixtures are also suitable.
In process variants a) and/or b), the polymerization according to the present
invention
should have been performed especially as a free-radical bulk polymerization,
solution
polymerization, gel polymerization, emulsion polymerization, dispersion
polymerization
or suspension polymerization. Gel polymerization has been found to be
particularly
suitable.
It is also advisable to perform the polymerization under adiabatic conditions,
in which
case the reaction should preferably have been started with a redox initiator
and/or a
photoinitiator.
Overall, the temperature is uncritical for the preparation of the
superabsorbent
polymers according to the present invention. However, it has been found to be
favourable not just owing to economic considerations when the polymerization
has
been started at temperatures between -20 and +60 C. Ranges between -10 and +50
C
and especially between 0 and 40 C and preferably more than 30 C have been
found to
be particularly suitable as start temperatures. With regard to the process
pressure too,
the present invention is not subject to any restriction. This is also the
reason why the
polymerization can ideally be performed under atmospheric pressure and,
overall,
without supplying any heat at all, which is considered to be an advantage of
the
present invention.
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9
The use of nonaqueous solvents is essentially not required either for the
polymerization
reaction. However, it may be found to be favourable in specific cases when the
preparation of the superabsorbent polymers has been performed in the presence
of at
least one water-immiscible solvent and especially in the presence of an
organic
solvent. In the case of the organic solvents, it should have been selected
from the
group of the linear aliphatic hydrocarbons and preferably n-pentane, n-hexane
and n-
heptane. However, branched aliphatic hydrocarbons (isoparaffins),
cycloaliphatic
hydrocarbons and preferably cyclohexane and decalin, or aromatic hydrocarbons,
and
here especially benzene, toluene and xylene, but also alcohols, ketones,
carboxylic
esters, nitro compounds, halogenated hydrocarbons, ethers, or any suitable
mixtures
thereof, are also useful. Organic solvents which form azeotropic mixtures with
water
are particularly suitable.
As already explained, the superabsorbent polymers according to the present
invention
are based on ethylenically unsaturated vinyl compounds. In this connection,
the
present invention envisages selecting these compounds from the group of the
ethylenically unsaturated, water-soluble carboxylic acids and ethylenically
unsaturated
sulphonic acid monomers, and salts and derivatives thereof, and preferably
acrylic
acid, methacrylic acid, ethacrylic acid, a-chloroacrylic acid, R-cyanoacrylic
acid, (i-
methylacrylic acid (crotonic acid), a-phenylacrylic acid, R-
acryloyloxypropionic acid,
sorbic acid, a-chlorosorbic acid, 2'-methylisocrotonic acid, cinnamic acid, p-
chlorocinnamic acid, itaconic acid, citraconic acid, mesaconic acid,
glutaconic acid,
aconitic acid, maleic acid, fumaric acid, tricarboxyethylene, maleic anhydride
or any
mixtures thereof.
A useful acryloyl- or methacryloylsulphonic acid is at least one
representative from the
group of sulphoethyl acrylate, sulphoethyl methacrylate, sulphopropyl
acrylate,
sulphopropyl methacrylate, 2-hydroxy-3-methacryloyloxypropylsulphonic acid and
2-
acrylamido-2-methylpropanesulphonic acid (AMPS).
Particularly suitable nonionic monomers should have been selected from the
group of
the water-soluble acrylamide derivatives, preferably alkyl-substituted
acrylamides or
aminoalkyl-substituted derivatives of acrylamide or of methacrylamide, and
more
preferably acrylamide, methacrylamide, N-methylacrylamide, N-
methylmethacrylamide,
N,N-dimethylacrylamide, N-ethylacrylamide, N,N-diethylacrylamide, N-
cyclohexylacrylamide, N-benzylacrylamide, N,N-dimethylaminopropylacrylamide,
N,N-
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dimethylaminoethylacrylamide, N-tert-butylacrylamide, N-vinylformamide, N-
vinylacetamide, acrylonitrile, methacrylonitrile, or any mixtures thereof.
Further suitable
monomers are, in accordance with the invention, vinyllactams such as N-
vinylpyrrolidone or N-vinylcaprolactam, and vinyl ethers such as
methylpolyethylene
5 glycol-(350 to 3000) monovinyl ether, or those which derive from
hydroxybutyl vinyl
ether, such as polyethylene glycol-(500 to 5000) vinyloxybutyl ether,
polyethylene
glycol-block-propylene glycol-(500 to 5000) vinyloxybutyl ether, though mixed
forms are
of course useful in these cases too.
10 The pure embodiments are described in detail hereinafter:
Variant a): combination of a first crosslinker, non-hydrolysable under the
conditions of
its application, and of a second crosslinker, showing a hydrolysable carbonic
acid ester
function under the conditions of its application.
In this pure embodiment a), the retardation is achieved by a specific
combination of the
crosslinkers. The combination of two or more crosslinkers in a superabsorbent
polymer
is nothing new per se. It is discussed in detail, for example, in US 5837789.
In the past,
the combination of crosslinkers has been used, however, in order to improve
the
antagonistic properties of absorption capacity and soluble fraction, and of
absorption
capacity and permeability. Specifically, a high absorption is promoted by
small amounts
of crosslinker; however, this leads to increased soluble fractions and vice
versa. The
combination of different crosslinkers forms, overall, better products over the
three
properties of absorption capacity, soluble fraction and permeability. The
retardation of
the swelling by several minutes by virtue of a crosslinker combination and
more
particularly to > 10 minutes has to date been unknown. When, for example, in
the area
of superabsorbent polymers for nappies, a time delay is established in order
that the
liquid is first distributed within the nappy and then absorbed, it is
typically in the region
of a few seconds.
Preferably, the inventive superabsorbents of this embodiment a) are present
either in
the form of anionic or cationic polyelectrolytes, but essentially not as
polyampholytes.
Polyampholytes are understood to mean polyelectrolytes which bear both
cationic and
anionic charges on the polymer chain. Preference is thus given in this case to
copolymers of purely anionic or purely cationic nature and not polyampholytes.
However, up to 10 mol%, preferably less than 5 mol%, of the total charge of a
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11
polyelectrolyte may be replaced by components of opposite charge. This applies
both
in the case of predominantly anionic copolymers with a relatively small
cationic
component and also conversely to predominantly cationic copolymers with a
relatively
small anionic fraction.
Suitable monomers for anionic superabsorbent polymers are, for example,
ethylenically
unsaturated, water-soluble carboxylic acids and carboxylic acid derivatives or
ethylenically unsaturated sulphonic acid monomers.
Preferred ethylenically unsaturated carboxylic acid or carboxylic anhydride
monomers
are acrylic acid, methacrylic acid, ethacrylic acid, a-chloroacrylic acid, a-
cyanoacrylic
acid, P-methylacrylic acid (crotonic acid), a-phenylacrylic acid, (3-
acryloyioxypropionic
acid, sorbic acid, a-chlorosorbic acid, 2'-methylisocrotonic acid, cinnamic
acid, p-
chlorocinnamic acid, itaconic acid, citraconic acid, mesaconic acid,
glutaconic acid,
aconitic acid, maleic acid, fumaric acid, tricarboxyethylene and maleic
anhydride,
particular preference being given to acrylic acid and methacrylic acid.
Ethylenically
unsaturated sulphonic acid monomers are preferably aliphatic or aromatic
vinylsulphonic acids or acrylic or methacrylic sulphonic acids. Preferred
aliphatic or
aromatic vinylsulphonic acids are vinyisulphonic acid, allylsulphonic acid,
vinyltoluenesulphonic acid and styrenesulphonic acid.
Preferred acryloyl- and methacryloylsulphonic acids are sulphoethyl acrylate,
sulphoethyl methacrylate, sulphopropyl acrylate, sulphopropyl methacrylate, 2-
hydroxy-
3-methacryloyloxypropylsulphonic acid and 2-acrylamido-2-
methylpropanesulphonic
acid, particular preference being given to 2-acrylamido-2-
methylpropanesulphonic acid.
All acids listed may have been polymerized as free acids or as salts. Of
course, partial
neutralization is also possible. In addition, some or all of the
neutralization may also be
effected only after the polymerization. The monomers can be neutralized with
alkali
metal hydroxides, alkaline earth metal hydroxides or ammonia. In addition, any
further
organic or inorganic base which forms a water-soluble salt with the acid is
conceivable.
Mixed neutralization with different bases is also conceivable. A preferred
feature of this
invention is neutralization with ammonia and alkali metal hydroxides, and more
preferably with sodium hydroxide.
In addition, further nonionic monomers with which the number of anionic
charges in the
CA 02703043 2010-05-13
12
polymer chain can be adjusted may also have been used. Possible water-soluble
acrylamide derivatives are alkyl-substituted acrylamides or aminoalkyl-
substituted
derivatives of acrylamide or of methacrylamide, for example acrylamide,
methacrylamide, N-methylacrylamide, N-methylmethacrylamide, N,N-
dimethylacrylamide, N-ethylacrylamide, N,N-diethylacrylamide, N-
cyclohexylacrylamide, N-benzylacrylamide, N,N-dimethylaminopropylacrylamide,
N,N-
dimethylaminoethylacrylamide and/or N-tert-butylacrylamide. Further suitable
nonionic
monomers are N-vinylformamide, N-vinylacetamide, acrylonitrile and
methacrylonitrile,
but also vinyllactams such as N-vinylpyrrolidone or N-vinylcaprolactam, and
vinyl
ethers such as methylpolyethylene glycol-(350 to 3000) monovinyl ether, or
those
which derive from hydroxybutyl vinyl ether, such as polyethylene glycol-(500
to 5000)
vinyloxybutyl ether, polyethylene glycol-block-propylene glycol-(500 to 5000)
vinyloxybutyl ether, and suitable mixtures thereof.
In addition, the inventive superabsorbent polymers comprise at least two
crosslinkers:
in general, a crosslinker forms a bond between two polymer chains, which leads
to the
superabsorbent polymers forming water-swellable but water-insoluble networks.
One
class of crosslinkers is that of monomers with at least two independently
incorporable
double bonds which lead to the formation of a network. In the context of the
present
invention, at least one crosslinker from the group of the crosslinkers not
hydrolysable
under the conditions of its application, and at least one crosslinker from the
group of
the crosslinkers hydrolysable under the conditions of its application, was
selected.
According to the invention, a non-hydrolysable crosslinker shall be understood
to mean
a crosslinker which, incorporated in the network, maintains its crosslinking
action at all
pH values; this means that under the conditions of the application (time,
temperature,
pH) there is nearly no hydrolysis. The linkage points of the network thus
cannot be
broken up by a change in the swelling medium. This contrasts with the
crosslinker
which under the conditions of the application is hydrolysable and which,
incorporated in
the network, can lose its crosslinking action through a change in the pH. One
example
of this is a diacrylate crosslinker which loses its crosslinking action
through alkaline
ester hydrolysis at a high pH.
Possible crosslinkers, not hydrolysable under the conditions of the
application, are
N,N'-methylenebisacrylamide, N,N'-methylenebismethacrylamide and monomers
having more than one maleimide group, such as hexamethylenebismaleimide;
monomers having more than one vinyl ether group, such as ethylene glycol
divinyl
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13
ether, triethylene glycol divinyl ether and/or cyclohexanediol divinyl ether.
It is also
possible to use allylamino or allylammonium compounds having more than one
allyl
group, such as triallylamine and/or tetraallylammonium salts. The crosslinkers
not
hydrolysable under the conditions of its application also include the allyl
ethers, such as
tetraallyloxyethane and pentaerythritol triallyl ether.
The group of the monomers having more than one vinylaromatic group includes
divinylbenzene and triallyl isocyanurate.
A preferred feature of the present invention is that, in process variant a),
the used
crosslinker, that is not hydrolysable under the conditions of its application,
was at least
one representative from the group of N,N'-methylenebisacrylamide, N,N'-
methylenebismethacrylamide or monomers having at least one maleimide group,
preferably hexamethylenebismaleimide, monomers having more than one vinyl
ether
group, preferably ethylene glycol divinyl ether, triethylene glycol divinyl
ether,
cyclohexanediol divinyl ether, allylamino or allylammonium compounds having
more
than one allyl group, preferably triallylamine or a tetraallylammonium salt
such as
tetraallylammonium chloride, or allyl ethers having more than one allyl group,
such as
tetraallyloxyethane and pentaerythritol triallyl ether, or monomers having
vinylaromatic
groups, preferably divinylbenzene and triallyl isocyanurate, or diamines,
triamines,
tetramines or higher-functionality amines, preferably ethylenediamine and
diethylenetriamine.
Crosslinkers with a hydrolysable carbonic acid ester function under the
conditions of its
application may be: poly-(meth)acryloyl-functional monomers, such as 1,4-
butanediol
diacrylate, 1,4-butanediol dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-
butylene
glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol
dimethacrylate,
ethylene glycol diacrylate, ethylene glycol dimethacrylate, ethoxylated
bisphenol A
diacrylate, ethoxylated bisphenol A dimethacrylate, 1,6-hexanediol diacrylate,
1,6-
hexanediol dimethacrylate, neopentyl glycol dimethacrylate, polyethylene
glycol
diacrylate, polyethylene glycol dimethacrylate, triethylene glycol diacrylate,
triethylene
glycol dimethacrylate, tripropylene glycol diacrylate, tetraethylene glycol
diacrylate,
tetraethylene glycol dimethacrylate, dipentaerythritol pentaacrylate,
pentaerythritol
tetraacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate,
trimethylolpropane trimethacrylate, cyclopentadiene diacrylate, tris(2-
hydroxyethyl)
isocyanurate triacrylate and/or tris(2-hydroxyethyl) isocyanurate
trimethacrylate;
monomers having more than one vinyl ester or allyl ester group with
corresponding
CA 02703043 2010-05-13
14
carboxylic acid, such as divinyl esters of polycarboxylic acids, diallyl
esters of
polycarboxylic acids, triallyl terephthalate, diallyl maleate, diallyl
fumarate, trivinyl
trimellitate, divinyl adipate and/or diallyl succinate.
The preferred representatives of crosslinkers with a hydrolysable carbonic
acid ester
function under the conditions of its application and used in preparation
variant a) were
compounds which were selected from the group of the di-, tri- or
tetra(meth)acrylates,
such as 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butylene
glycol
diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate,
diethylene
glycol dimethacrylate, ethylene glycol diacrylate, ethylene glycol
dimethacrylate,
ethoxylated bisphenol A diacrylate, ethoxylated bisphenol A dimethacrylate,
1,6-
hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol
dimethacrylate,
polyethylene glycol diacrylate, polyethylene glycol dimethacrylate,
triethylene glycol
diacrylate, triethylene glycol dimethacrylate, tripropylene glycol diacrylate,
tetraethylene
glycol diacrylate, tetraethylene glycol dimethacrylate, dipentaerythritol
pentaacrylate,
pentaerythritol tetraacrylate, pentaerythritol triacrylate, trimethylolpropane
triacrylate,
trimethylolpropane trimethacrylate, cyclopentadiene diacrylate, tris(2-
hydroxyethyl)
isocyanurate triacrylate and/or tris(2-hydroxyethyl) isocyanurate
trimethacrylate, the
monomers having more than one vinyl ester or allyl ester group with
corresponding
carboxylic acids, such as divinyl esters of polycarboxylic acids, diallyl
esters of
polycarboxylic acids, diallyl maleate, diallyl fumarate, trivinyl
trimellitate, divinyl adipate
and/or diallyl succinate, or at least one representative of the compounds
having at least
one vinylic or allylic double bond and at least one epoxy group, such as
glycidyl
acrylate, allyl glycidyl ether, or the compounds having more than one epoxy
group,
such as ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether,
polyethylene
glycol diglycidyl ether, polypropylene glycol diglycidyl ether, or the
compounds having
at least one vinylic or allylic double bond and at least one (meth)acrylate
group, such
as polyethylene glycol monoallyl ether acrylate or polyethylene glycol
monoallyl ether
methacrylate.
Further crosslinkers which contain functional groups both from the class of
the carbonic
acid ester functions hydrolysable under the conditions of the application and
of the
crosslinker groups not hydrolysable under the conditions of the application
should be
included among the class of crosslinkers that show a hydrolysable carbonic
acid ester
function under the conditions of the application, when they form not more than
one
crosslinking point not hydrolysable under the conditions of the application.
Typical
CA 02703043 2010-05-13
examples of such crosslinkers are polyethylene glycol monoallyl ether acrylate
and
polyethylene glycol monoallyl ether methacrylate.
In addition to the crosslinkers having two or more double bonds, there are
also those
which have only one or no double bond, but do have other functional groups
which can
5 react with the monomers and which lead to crosslinking points during the
preparation
process. Two frequently used functional groups are in particular epoxy groups
and
amino groups. Examples of such crosslinkers with a double bond are glycidyl
acrylate,
allyl glycidyl ether. Examples of crosslinkers without a double bond are
diamines,
triamines or compounds having four or more amino groups, such as
ethylenediamine,
10 diethylenetriamine, or diepoxides such as ethylene glycol diglycidyl ether,
diethylene
glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene
glycol
diglycidyl ether.
In the preparation of the inventive superabsorbents, sufficiently high total
amounts of
15 crosslinker as to give rise to a very close-mesh network are typically
used. The
polymeric product thus has only a low absorption capacity after short times (>
5 min; <
10 min).
The amounts of the crosslinkers not hydrolysable under the conditions of the
application used in process variant a) were between 0.01 and 1.0 mol%,
preferably
between 0.03 and 0.7 mol% and more preferably 0.05 to 0.5 mol%. Significantly
higher
amounts of the crosslinkers showing a hydrolysable carbonic acid ester
function under
the conditions of the application are required: according to the invention,
0.1 to
10.0 mol%, preferably 0.3 to 7 mol% and more preferably 0.5 to 5.0 mol% were
used.
Under the use conditions preferred in accordance with the invention, the
hydrolysis-
labile network links formed in the course of polymerization are broken again.
The
absorption capacity of the inventive superabsorbent polymer is increased
significantly
as a result. The required amounts of the crosslinkers should, though, be
adjusted to the
particular application and should be determined in performance tests (for
construction
material systems particularly in the time-dependent slump).
Cationic superabsorbent polymers contain exclusively cationic monomers. For
cationic
superabsorbent polymers of embodiment a), it is possible to use all monomers
with a
permanent cationic charge. "Permanent" means in turn that the cationic charge
remains predominantly stable in an alkaline medium; an ester quat is, for
example,
CA 02703043 2010-05-13
16
unsuitable. The nonionic comonomers and crosslinkers used may be all monomers
listed among the anionic superabsorbent polymers, employing the abovementioned
molar ratios. Possible cationic monomers are:
[3-(acryloylamino)propyl]trimethylammonium salts and/or
[3-(methacryloylamino)propyl]trimethylammonium salts. The salts mentioned are
preferably present in the form of halides, sulphates or methosulphates. In
addition, it is
possible to use diallyidimethylammonium chloride.
The inventive anionic or cationic superabsorbent copolymers can be prepared in
a
manner known per se by joining the monomers which form the particular
structural
units by free-radical polymerization. All monomers present in acid form can be
polymerized as free acids or in the salt form thereof. In addition, the acids
can be
neutralized by adding appropriate bases even after the copolymerization;
partial
neutralization before or after the polymerization is likewise possible. The
monomers or
the copolymers can be neutralized, for example, with the bases sodium
hydroxide,
potassium hydroxide, calcium hydroxide, magnesium hydroxide and/or ammonia.
Likewise suitable as bases are C,- to C2o-alkylamines, C,- to C2o-
alkanolamines, C5- to
C8-cycloalkylamines and/or C6- to C,a-arylamines, each of which has primary,
secondary or tertiary and in each case branched or unbranched alkyl groups. It
is
possible to use one base or a plurality. Preference is given to neutralization
with alkali
metal hydroxides and/or ammonia; sodium hydroxide is particularly suitable.
The
inorganic or organic bases should be selected such that they form readily
water-soluble
salts with the particular acid.
For all aminic bases and ammonia, it should be checked in the application
whether the
alkaline medium which is formed by the pore water forms a fishy and/or
ammoniacal
odour, since this may possibly be a criterion for exclusion.
As likewise already mentioned in general terms, the monomers should preferably
be
copolymerized by free-radical bulk polymerization, solution polymerization,
gel
polymerization, emulsion polymerization, dispersion polymerization or
suspension
polymerization. Since the inventive products are hydrophilic and water-
swellable
copolymers, polymerization in aqueous phase, polymerization in inverse
emulsion
(water-in-oil) and polymerization in inverse suspension (water-in-oil) are
preferred
variants. In particularly preferred embodiments, the reaction is effected as a
gel
polymerization or else as an inverse suspension polymerization in organic
solvents.
CA 02703043 2010-05-13
17
Process variant a) may also have been performed as an adiabatic
polymerization, and
may have been started either with a redox initiator system or with a
photoinitator.
However, a combination of both variants of the initiation is also possible.
The redox
initiator system consists of at least two components, an organic or inorganic
oxidizing
agent and an organic or inorganic reducing agent. Frequently, compounds with
peroxide units are used, for example inorganic peroxides such as alkali metal
persulphate and ammonium persulphate, alkali metal perphosphates and ammonium
perphosphates, hydrogen peroxide and salts thereof (sodium peroxide, barium
peroxide), or organic peroxides such as benzoyl peroxide, butyl hydroperoxide,
or
peracids such as peracetic acid. In addition, it is also possible to use other
oxidizing
agents, for example potassium permanganate, sodium chlorate and potassium
chlorate, potassium dichromate, etc. The reducing agents used may be sulphur
compounds such as sulphites, thiosulphates, sulphinic acid, organic thiols
(for example
ethyl mercaptan, 2-hydroxyethanethiol, 2-mercaptoethylammonium chloride,
thioglycolic acid) and others. In addition, ascorbic acid and low-valency
metal salts
[copper(l); manganese(II); iron(II)] are suitable. Phosphorus compounds, for
example
sodium hypophosphite, can also be used. As their name suggests,
photopolymerizations are started with UV light, which results in the
decomposition of a
photoinitiator. The photoinitiators used may, for example, be benzoin and
benzoin
derivatives, such as benzoin ethers, benzil and derivatives thereof, such as
benzil
ketals, aryldiazonium salts, azo initiators, for example 2,2'-
azobis(isobutyronitrile), 2,2'-
azobis(2-amidinopropane) hydrochloride and/or acetophenone derivatives. The
proportion by weight of the oxidizing component and of the reducing component
in the
case of the redox initiator systems is preferably in each case in the range
between
0.00005 and 0.5% by weight, more preferably in each case between 0.001 and 0.1
%
by weight. For photoinitiators, this range is preferably between 0.001 and 0.1
% by
weight and more preferably between 0.002 and 0.05% by weight. The percentages
by
weight stated for the oxidizing and reducing components and the
photoinitiators are
based in each case on the mass of the monomers used for the copolymerization.
The
polymerization conditions, especially the amounts of initiator, are always
selected with
the aim of obtaining very long-chain polymers. Owing to the insolubility of
the
crosslinked copolymers, the determination of the molecular weights is,
however,
possible only with great difficulty.
The copolymerization is preferably performed in aqueous solution, especially
in
concentrated aqueous solution, batchwise in a polymerization vessel (batchwise
process) or continuously by the "endless belt" method described, for example,
in US-A-
CA 02703043 2010-05-13
18
4857610. A further possibility is polymerization in a continuous or batchwise
kneading
reactor. The process is started typically at a temperature between -20 and 20
C,
preferably between -10 and 10 C, and performed at atmospheric pressure and
without
external heat supply, the heat of polymerization resulting in a maximum end
temperature, depending on the monomer content, of 50 to 150 C. The end of the
copolymerization is generally followed by comminution of the polymer present
in gel
form. In the case of performance on the laboratory scale, the comminuted gel
is dried
in a forced-air drying cabinet at 70 to 180 C, preferably at 80 to 150 C. On
the
industrial scale, the drying can also be effected in a continuous manner
within the
same temperature ranges, for example on a belt dryer or in a fluidized bed
dryer. In a
further preferred embodiment, the copolymerization is effected as an inverse
suspension polymerization of the aqueous monomer phase in an organic solvent.
The
procedure here is preferably to polymerize the monomer mixture which has been
dissolved in water and optionally neutralized in the presence of an organic
solvent in
which the aqueous monomer phase is soluble sparingly, if at all. Preference is
given to
working in the presence of "water-in-oil" emulsifiers (W/O emulsifiers) and/or
protective
colloids based on low or high molecular weight compounds which are used in
proportions of 0.05 to 5% by weight, preferably 0.1 to 3% by weight (based in
each
case on the monomers). The W/O emulsifiers and protective colloids are also
referred
to as stabilizers. It is possible to use customary compounds known as
stabilizers in
inverse suspension polymerization technology, such as hydroxypropylcellulose,
ethylcellulose, methylcellulose, cellulose acetate butyrate mixed ethers,
copolymers of
ethylene and vinyl acetate, of styrene and butyl acrylate, polyoxyethylene
sorbitan
monooleate, monolaurate or monostearate, and block copolymers of propylene
oxide
and/or ethylene oxide. Suitable organic solvents are, for example, linear
aliphatic
hydrocarbons such as n-pentane, n-hexane, n-heptane, branched aliphatic
hydrocarbons (isoparaffins), cycloaliphatic hydrocarbons such as cyclohexane
and
decalin, and aromatic hydrocarbons such as benzene, toluene and xylene.
Further
suitable solvents are alcohols, ketones, carboxylic esters, nitro compounds,
halogenated hydrocarbons, ethers and many other organic solvents. Preference
is
given to organic solvents which form azeotropic mixtures with water,
particular
preference to those which have a very high water content in the azeotrope.
The water-swellable copolymers (superabsorbent precursor) are initially
obtained in
swollen form as finely distributed aqueous droplets in the organic suspension
medium,
and are preferably isolated as solid spherical particles in the organic
suspension
medium by removing the water by azeotropic distillation. Removal of the
suspension
CA 02703043 2010-05-13
19
medium and drying leaves a pulveruient solid. Inverse suspension
polymerization is
known to have the advantage that variation of the polymerization conditions
allows the
particle size distribution of the powders to be controlled. An additional
process step
(grinding operation) to adjust the particle size distribution can usually be
avoided as a
result.
The monomers and crosslinkers should be selected taking account of the
particular
requirements, some of them specific, of the application. For instance, in the
case of
high salt burdens in the construction material system, salt-stable monomer
compositions should be employed, which may be based, for example, on sulphonic
acid-based monomers. In this case, the final absorption is established via the
monomer
composition and the crosslinkers not hydrolysable under the conditions of the
application, while the crosslinker showing a hydrolysable carbonic acid ester
function
under the conditions of the application influences the kinetics of the
swelling. However,
it should be taken into account that the monomer composition and the
crosslinker can
also have a certain influence on the kinetics, which is different from case to
case and,
in particular, is less marked with respect to the influence of the crosslinker
showing a
hydrolysable carbonic acid ester function under the conditions of the
application. Both
the crosslinker not hydrolysable under the conditions of the application and
the
crosslinker showing a hydrolysable carbonic acid ester function under the
conditions of
the application should, according to the invention, be incorporated
homogeneously.
Otherwise, for example, regions depleted of crosslinker showing a hydrolysable
carbonic acid ester function under the conditions of the application would
form and
would therefore begin to swell rapidly, without exhibiting the desired time
delay. Too
high a reactivity of the crosslinker can lead to it already being consumed
before the end
of the polymerization, and no further crosslinker is available at the end of
the
polymerization. Too low a reactivity has the effect that, at the start of the
polymerization, regions low in crosslinker are formed. In addition, there is
always the
risk that the second double bond is not incorporated fully - the crosslinking
function
would thus be absent. The length of the bridge between the crosslinking points
may
likewise have an influence on the hydrolysis kinetics. Steric hindrance can
slow the
hydrolysis. Overall, the selection of the composition of the superabsorbent
polymer is
influenced by the application (construction material system and time window
for the
hydrolysis). However, the present invention provides sufficient possible
variations and
selections, and so it is possible without any problems to select suitable
crosslinkers of
mentioned classes, for example in order to ensure a homogeneous network.
Variant b): combination of a permanently anionic monomer with a hydrolysable
cationic
monomer, suitable for the release of its cationic charge at a pH >7 by ester
hydrolysis
and/or a deprotonation.
CA 02703043 2010-05-13
In this second embodiment, the time delay of the swelling action of the SAP is
achieved
through a specific combination of the monomers.
5 The superabsorbents of this embodiment b) of the invention are present in
the form of
polyampholytes. Polyampholytes are understood to mean polyelectrolytes which
bear
both cationic and anionic charges on the polymer chain. Combination of
cationic and
anionic charge within the polymer chain results in formation of strong
intramolecular
attraction forces which lead to the absorption capacity being reduced
significantly, or
10 even approaching zero.
In embodiment b), the cationic monomers were selected such that they lose
their
cationic charge with time and become uncharged or even anionic. The two
following
reaction schemes are intended to illustrate this in detail:
In the first case, a cationic ester quat, as a polymerized constituent of the
SAP, is
15 converted in the course of application by an alkaline hydrolysis to a
carboxylate.
In the second case, a cationic acrylamide derivative becomes nonionic as a
result of a
neutralization.
O O
Case 1 + H Me
0 Me OH 0 ~N Me
-\_N Me Me
I
Me
caticxnic anionic
Case 2
0 0
H Me H
N~ Me\N~Me OH N~N-Me + 20
H H
cationic non-icmic
Useful anionic monomers in this process variant b) are all anionic monomers
already
mentioned for process variant a). Preferred representatives in accordance with
the
invention are considered to be those from the group of the ethylenically
unsaturated
CA 02703043 2010-05-13
21
water-soluble carboxylic acids and ethylenically unsaturated sulphonic acid
monomers,
and salts and derivatives thereof, especially acrylic acid, methacrylic acid,
ethacrylic
acid, a-chloroacrylic acid, a-cyanoacrylic acid, P-methylacrylic acid
(crotonic acid),
a-phenylacrylic acid, R-acryloyloxypropionic acid, sorbic acid, a-chlorosorbic
acid,
2'-methylisocrotonic acid, cinnamic acid, p-chlorocinnamic acid, itaconic
acid, citraconic
acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fumaric
acid,
tricarboxyethylene and maleic anhydride, more preferably acrylic acid,
methacrylic
acid, aliphatic or aromatic vinylsulphonic acids, and especially preferably
vinylsulphonic
acid, allylsulphonic acid, vinyltoluenesulphonic acid, styrenesulphonic acid,
acryloyl-
and methacryloylsulphonic acids, and even more preferably sulphoethyl
acrylate,
sulphoethyl methacrylate, sulphopropyl acrylate, suiphopropyl methacrylate, 2-
hydroxy-
3-methacryloyloxypropylsulphonic acid and 2-acrylamido-2-
methylpropanesulphonic
acid (AMPS), or mixtures thereof.
Representatives of cationic monomers suitable for the release of its cationic
charge by
ester hydrolysis and/or deprotonation under the conditions of the application
may be:
- For Case 1 in Figure 1: [2-(acryloyloxy)ethyl]trimethylammonium salts and [2-
(methacryloyloxy)ethyl]trimethylammonium salts. In principle, all
polymerizable cationic
esters of vinyl compounds whose cationic charge can be eliminated by
hydrolysis are
conceivable.
- For Case 2 in Figure 1: salts of 3-dimethylaminopropylacrylamide or 3-
dimethylaminopropylmethacrylamide, preference being given to the hydrochloride
and
hydrosulphate. In principle, all monomers which are vinylically polymerizable
and bear
an amino function which can be protonated can be used. Preferred
representatives of
the cationic monomers are, according to the present invention, polymerizable
cationic
esters of vinyl compounds whose cationic charge can be eliminated by
hydrolysis,
preferably [2-(acryloyloxy)ethyl]trimethylammonium salts and [2-
(methacryloyloxy)ethyl]trimethylammonium salts, or monomers which are
vinylically
polymerizable and bear an amino function which can be protonated, preferably
salts of
3-dimethylaminopropylacrylamide or 3-dimethylaminopropylmethacrylamide, and
more
preferably the hydrochloride and hydrosulphate thereof, or mixtures thereof.
Since the inventive SAPs prepared by process variant b) are suitable in
particular for
applications having a high pH, which is the case especially in cementitious
systems, at
least one crosslinker should be selected from the above-described group of the
CA 02703043 2010-05-13
22
crosslinkers not hydrolysable under the conditions of the application.
The present invention also envisages that the SAPs can be prepared by all
variants as
have already been described under embodiment a).
To control the retardation, it is possible in principle to incorporate
additional monomers
from the group of the above-described nonionic monomers into the inventive
superabsorbent polymer. The use of nonionic monomers brings about an
acceleration
of the increase in the absorption capacity.
For the second process variant b) of the invention too, it is important first
to achieve an
absorption of close to zero in demineralized water. This is achieved through
the
selection of the correct amounts of cationic and anionic monomers. Ideally,
the
minimum absorption is achieved at a molar ratio of the cationic to anionic
monomers of
1:1. In the case of weak acids or bases, it may be necessary to establish a
molar ratio
which deviates from 1:1 (for example 1.1 to 2.0 : 2.0 to 1.1).
If relatively fast retarded swelling is required, a low absorption can also be
established.
This too is achieved by a monomer composition deviating from the ratio of 1:1
(for
example 1.1 to 2.0:2.0 to 1.1). As a result of the low residual absorption,
the retarded
superabsorbent polymer absorbs a little water or aqueous solution in the
application,
and the neutralization/hydrolysis takes place more rapidly. In all cases of
process
variant b), the molar ratio of anionic to cationic monomer is 0.3 to 2.0:1.0,
preferably
0.5 to 1.5:1.0 and more preferably 0.7 to 1.3:1Ø
A further means in principle of controlling the kinetics is the addition of
salt.
Polyampholytes often have an inverse electrolyte effect, i.e. the addition of
salts
increases the solubility in water. This salt is added to the monomer solution.
In the case
of gel polymerization, it may, though, also be added to the gel as an aqueous
solution.
The selection of the crosslinkers likewise allows the kinetics of the swelling
to be
influenced. The type and the amount of crosslinker are additionally crucial
for the
absorption behaviour of the retarded superabsorbent polymer after the complete
hydrolysis/neutralization of the cationic monomers. Again, the swelling
kinetics and the
final absorption should be and can be adjusted to the particular application.
In this
case, both the application and the raw materials of the formulation again play
a major
CA 02703043 2010-05-13
23
role.
A further possible variant of this embodiment is that of the so-called
interpenetrating
network: in this case, two networks are formed within one another. One network
is
formed from a polymer of cationic monomers, the second from anionic monomers.
The
charges should balance overall. It may be found to be favourable to
additionally
incorporate nonionic monomers into the network. Interpenetrating networks are
prepared by initially charging a cationic (or anionic) polymer in an anionic
(or cationic)
monomer solution and then polymerizing. The crosslinking should be selected
such
that the two polymers form a network: the initially charged polymer and the
newly
formed polymer.
Variant c: coating with an oppositely charged solution polymer
In this third process variant c), the retardation is achieved through a
specific surface
treatment of the superabsorbent polymer. In this case, the charged
superabsorbent
polymer is coated with an oppositely charged polymer. The balancing of the
charges on
the polymer surface, as preferably provided by the present invention, forms a
water-
impermeable simplex layer which prevents swelling of the superabsorbent
polymer
within the first few minutes. This surface treatment should become detached
from the
SAP with time (at least 10 to 15 minutes!), which significantly increases the
absorption
capacity of the superabsorbent polymer.
The surface treatment of anionic superabsorbent polymers, preferably
crosslinked,
partly neutralized polyacrylic acids, with cationic polymers has already been
described
in a series of patents: The already cited publications WO 2006/082188 and
WO 2006/082189 describe surface treatment with one to two percent of
polyamine; in
DE 10 2005 018922, polyDADMAC (polydiallyldimethylammonium chloride) is
applied
to superabsorbent polymers. In the course of polyamine coating, crosslinking
components are present. This involves spraying cationic polymers as aqueous
solutions onto the granular superabsorbent polymer. The superabsorbent
polymers
thus obtained have a higher permeability and a lower tendency to form lumps in
the
course of storage, i.e. remain free-flowing for longer. Since these SAPs have
been
developed exclusively for use in nappies, they of course must not have a time
delay in
the range of minutes. EP 1 393 757 Al describes surface coating with partly
hydrolysed polyvinylformamide. This leads to improved performance in the
nappy.
CA 02703043 2010-05-13
24
WO 2003/43670 likewise describes the crosslinking of polymers which have been
applied to the surface.
Generally, in accordance with the invention, cationic polymers with a
molecular weight
of 5 million g/mol or less are used, which, as a 10 to 20% aqueous solution,
give rise to
a sprayable solution (viscosity). They are polymerized as an aqueous solution
and
used for surface treatment. In the standard processes, the superabsorbent
polymer is
initially charged, for example in a fluidized bed, and sprayed with a polymer
solution.
Generally, "highly cationic" polymers are used, i.e. those whose cationic
monomers
make up at least 75 mol% of the composition.
The present invention prefers the use of shell polymers with a molecular
weight of
<_ 3 million g/mol, preferably _ 2 million g/mol and more preferably < 1.5
million g/mol,
and the selected shell polymers should have either anionic or cationic
properties.
Ampholytes are not used.
A further combination of cationic and anionic polyelectrolytes is that of MBIE-
superabsorbent polymers, where MBIE stands for "mixed bed ion exchange". Such
products are described, inter alia, in US 6,603,056 and the patents cited
there: a
potentially anionic superabsorbent polymer is mixed with a superabsorbent
cationic
polymer. "Potentially anionic" means that, in the embodiments of the
invention, the
anionic superabsorbent polymer is used in acidic form. While the purely
anionic
superabsorbent polymers are usually polyacrylic acids neutralized to an extent
of
approx. 70%, crosslinked polyacrylic acids which are neutralized only to a low
degree,
if at all, are used here. The combination with a cationic polymer leads to a
more salt-
stable product; the salts are effectively neutralized by ion exchange, as
shown in
Figure 2 below. The neutralized acid then possesses the appropriate osmotic
pressure
(n) for significant swelling.
O O
H2 + NaCi > H3+CI
OH ONa
This concept for superabsorbent polymers was also developed exclusively for
use in
CA 02703043 2010-05-13
hygiene articles, specifically in nappies, and is thus again aimed at fast
superabsorbent
polymers. The combination of anionic and cationic superabsorbent polymer to
provide
a superabsorbent polymer retarded in the range of minutes has not been
described to
date.
5
The starting material used for the surface treatment in the present invention
may be
any superabsorbent polymer which has sufficient absorption capacity in
cementitious
systems in particular. It may be either anionic or cationic. The starting
material shall be
referred to hereinafter as "core polymer". The polymer which is applied to the
surface
10 shall be referred to hereinafter as "shell polymer". The core polymers are
anionic or
cationic superabsorbent polymers, preferably in the sense of process variant
a), which
have especially 5 10% by weight of comonomers with opposite charge. In
contrast to
variant a), the core polymers used in pure embodiment c) are, however, only
superabsorbent polymers which are formed exclusively from crosslinkers not
15 hydrolysable under the conditions of the application. This variant is
considered to be
preferred. Apart from the restriction for the crosslinkers, the synthesis of
the anionic
core polymers corresponds to that described in process variant a). For the
present
case too, it is possible to use all monomers already described there.
20 For cationic core polymers, it is possible to use all monomers with a
permanent cationic
charge. "Permanent" in turn means that the cationic charge is maintained in
alkaline
medium and thereby stable under the conditions of the application; an ester
quat is
thus unsuitable. Preference is given to: [3-
(acryloylamino)propyl]trimethylammonium
salts and [3-(methacryloylamino)propyl]trimethylammonium salts. The salts
mentioned
25 are preferably present as halides, methosulphates or sulphates. In
addition, it is
possible to use diallyidimethylammonium chloride.
For the treatment of the surface, two preferred processes are possible, both
of which
are also described in US 6,603,056:
One process is basically a conventional powder coating. The core polymer is
initially
charged and set in motion, for example in a fluidized bed. Subsequently, the
oppositely
charged shell polymer is applied. Finally, the product is dried. This process
is suitable
in particular when relatively small amounts of shell polymer based on the core
polymer
are to be applied. In the case of larger amounts in this process,
conglutination of the
particles occurs and the product cakes together. This leads to the surfaces no
longer
being coated homogeneously. In order to apply large amounts of shell polymer,
this
process step has to be carried out repeatedly.
CA 02703043 2010-05-13
26
For larger amounts of shell polymer, a second process is suitable: in this
process, the
core polymer is suspended in an organic solvent. The shell polymer solution is
added
to the suspension, and then, for electrostatic reasons, the core polymer is
coated with
an oppositely charged shell. For very small particles too, this process is
advantageous
since they are difficult to handle in a fluidized bed.
After the addition of the shell polymer solution, the amount of water added
through the
solution can optionally be distilled off azeotropically. Therefore, preferred
organic
solvents are considered to be those which form an azeotrope with a maximum
water
content, in which the superabsorbent polymer and the shell polymer are
insoluble. For
this process, it is possible to use the same solvents which are also specified
in process
variant a) among the solvents for the suspension polymerization. It has also
been
found to be advantageous to add a protective colloid, as is also done in the
suspension
polymerization. Again, it is possible to select from the protective colloids
described
there.
For the surface coating, as described, a shell polymer is applied to the core
polymer.
The shell polymer is preferably applied as an aqueous solution and is
especially used
as a sprayable solution, particularly suitable solutions being those having a
viscosity of
from 200 to 7500 mPas. Working with organic solvents is very complicated in
this
process, particularly on the industrial scale. For both processes just
described, it is
favourable to work with low-viscosity solutions since they can be sprayed
better and
also become attached more readily to the surface of the suspended core
polymer.
Since the molecular weight of the shell polymer has a significant influence on
the
viscosity, shell polymers with a molecular weight of less than 5 million g/mol
are
preferred. Moreover, it is envisaged in accordance with the invention that the
further
polyelectrolyte, i.e. the shell polymer, has a proportion of cationic monomer
of
z 75 mol%, preferably z 80 mol% and more preferably between 80 and 100 mol%.
In principle, it is possible to prepare such cationic or anionic shell
polymers either by
the process of gel polymerization or by that of suspension polymerization, and
then to
redissolve the resulting polymers and to apply them as an aqueous shell
polymerization solution. However, it is more advantageous to perform the
polymerization as a solution polymerization, such that the product of the
polymerization
CA 02703043 2010-05-13
27
can be used directly and no more than a dilution is still necessary. The
molecular
weight of the shell polymers can be reduced by the addition of chain
regulators, which
allows the desired chain length and hence also the desired viscosity to be
obtained.
The procedure is preferably as follows:
The monomers are dissolved in water or their commercially obtainable aqueous
solutions are diluted. Then the chain regulator(s) is/are added and the pH is
adjusted.
Subsequently, the aqueous monomer solution is inertized with nitrogen and
heated to
the start temperature. With the addition of the initiators, the polymerization
is started
and proceeds generally within a few minutes. The concentration of the shell
polymer is
selected at a maximum level in order that the amount of water to be removed is
at a
minimum, but the viscosity can still be handled readily in the processes
according to
the invention, such as spraying, coating in suspension. It may be advantageous
to heat
the shell polymer solution since the viscosity at the same concentration falls
at higher
temperatures. Suitable chain regulators are formic acid or salts thereof, for
example
sodium formate, hydrogen peroxide, compounds which comprise a mercapto group
(R-
SH) or a mercaptate group (R-S-M+), where the R radical here may in each case
be an
organic aliphatic or aromatic radical having 1 to 16 carbon atoms (for example
mercaptoethanol, 2-mercaptoethylamine, 2-mercaptoethylammonium chloride,
thioglycolic acid, mercaptoethanesulphonate (sodium salt), cysteine,
trismercaptotriazole (TMT) as the sodium salt, 3-mercaptotriazole, 2-mercapto-
l-
methylimidazole), compounds which comprise an R-S-S-R' group (disulphite
group),
where the R and R' radicals here may each independently be an organic
aliphatic or
aromatic radical having 1 to 16 carbon atoms (for example cystaminium
dichloride,
cysteine), phosphorus compounds, such as hypophosphorous acid and salts
thereof
(e.g. sodium hypophosphite), or sulphur-containing inorganic salts such as
sodium
sulphite.
Possible shell polymers for anionic core polymers are cationic polymers which
can lose
their cationic charge through a chemical reaction. Possible cationic monomers
for this
embodiment are ester quats, for example [2-
(acryloyloxy)ethyl]trimethylammonium
salts, [2-(methacryloyloxy)ethyl]trimethylammonium salts, dimethylaminoethyl
methacrylate quaternized with diethyl sulphate or dimethyl sulphate,
diethylaminoethyl
acrylate quaternized with methyl chloride. In this case, the chemical reaction
which
leads to retarded swelling of the SAP is an ester hydrolysis. A neutralization
reaction of
the shell polymer is possible with the following polymers: poly-3-
dimethylaminopro-
pylacrylamide, poly-3-dimethylaminopropylmethacrylamide, polyallylamine,
CA 02703043 2010-05-13
28
polyvinylamine, polyethyleneimine. All polymers are used here in the form of
salts. For
the neutralization of the amino function, inorganic or organic acids can be
used, and
their mixed salts are also suitable. All variants mentioned are encompassed by
the
present invention.
For the establishment of the kinetics of the detachment reaction which are
appropriate
for the application, it may be necessary to incorporate further nonionic
monomers into
the cationic shell polymer. It is possible to use all nonionic monomers
already
mentioned under process variant a).
This variant c) of the invention is not just restricted to one-layer shells.
In order to
achieve a further or more exact time delay, it is possible, after the first
shell layer which
has been applied directly to the core polymer, to apply a second with the same
charge
that the core polymer also originally possesses. This can be continued
further, in which
case the charges of the shell polymers alternate. An anionic core polymer
would be
followed after the first cationic shell by an anionic second shell. The third
shell would
then be cationic again. Irrespective of the number of different shell layers,
one or more
shell layer(s) may be crosslinked. Moreover, preferably at least one shell
layer should
have been crosslinked with the aid of an aqueous solution.
Moreover, the present invention takes account of the possibility that the
shell polymer
in process variant c), per layer applied, was used in an amount of 5 to 100%
by weight,
preferably of 10 to 80% by weight and more preferably in an amount of 25 to
75% by
weight, based in each case on the core polymer.
A further variation of the invention relates to the crosslinking of the shell
polymer and
the control of its detachment rate. To this end, it is possible, for example,
to use free
amino groups of the shell polymers. The crosslinker is added later than the
shell
polymer, preferably as an aqueous solution. In order to ensure full reaction
of the
crosslinker, it may be necessary to heat the retarded superabsorbent polymer
once
again after drying, or to perform the drying at elevated temperature. Possible
crosslinkers for this form of the procedure are diepoxides such as diethylene
glycol
diglycidyl ether or polyethylene glycol diglycidyl ether, diisocyanates (which
have to be
applied in anhydrous form after the drying), glyoxal, glyoxylic acid,
formaldehyde,
formaldehyde formers and suitable mixtures.
In order to control the kinetics of the detachment operation, the composition
of the shell
CA 02703043 2010-05-13
29
polymer should be adjusted to the core polymer. This can be done, for example,
by
determining the appropriate composition. It has been found to be favourable to
establish identical molar ratios in the core polymer and in the shell polymer;
however,
the charges must be different. According to the application, however,
deviations from
the molar ratios may also be found to be positive.
The optimal amount of shell polymer likewise has to be determined. Generally,
it can
be stated that finely structured core polymers require larger amounts of shell
polymer,
since they possess a greater surface area. The molecular weight of the shell
polymers
may also play a role, since short-chain shell polymers become detached more
readily.
The process of surface coating c) requires more process steps than the two
alternative
steps a) and b). In principle, it is also conceivable to perform the core
polymer
synthesis as an inverse suspension polymerization and, after the drying by
azeotropic
distillation, to supply a new monomer solution which corresponds to that of
the shell
polymer. Were this to be surface polymerized, process variant c) would be
reduced to a
one-pot reaction. However, the residence time in the reactor would be quite
long and it
is not easy to form a homogeneous layer of the shell polymer only at the
surface.
Variant d: combination of a first monomer, not hydrolysable under the
conditions of the
application, with a second monomer showing a hydrolysable carbonic acid ester
function under the conditions of the application, in the presence of a
crosslinker.
The further process variant d) of the invention relates to an SAP which, after
the
polymerization, is composed of at least two nonionic comonomers but contains
not
more than 5 mol% of anionic or cationic charge. Among these nonionic
comonomers is
at least one which can be converted by a chemical reaction, preferably a
hydrolysis, to
an ionic monomer which is defined in the following as monomer showing a
hydrolysable carbonic acid ester function under the conditions of the
application. The
remainder consists of permanently nonionic monomers which are not subject to
any
significant hydrolysis under the conditions of the application even in the
case of
prolonged treatment of the SAP at high pH. This monomer which is then ionic
gives
rise to an osmotic pressure which leads to greater swelling of the SAP. An
example
given is that of an SAP which consists of acrylamide and hydroxypropyl
acrylate (HPA),
and also a crosslinker. When this SAP is exposed to an alkaline medium, an
ester
hydrolysis of the HPA occurs, which leads to acrylate units. This gives rise
to an
additional osmotic pressure and the SAP swells further. In this embodiment, it
should
be noted that purely nonionic SAP also has a certain "natural" swelling
(entropy effect,
comparable to an EPDM rubber in petroleum); there is therefore not zero
swelling here
in the initial stage.
CA 02703043 2010-05-13
The polymerization is performed as already described in embodiment a).
Suitable monomers not hydrolysable under the conditions of the application are
5 preferably permanently nonionic monomers which are preferably selected from
the
group of the water-soluble acrylamide derivatives, preferably alkyl-
substituted
acrylamides or aminoalkyl-substituted derivatives of acrylamide or of
methacrylamide,
and more preferably acrylamide, methacrylamide, N-methylacrylamide, N-
methylmethacrylamide, N,N-dimethylacrylamide, N-ethylacrylamide, N,N-
10 diethylacrylamide, N-cyclohexylacrylamide, N-benzylacrylamide, N,N-
dimethylaminopropylacrylamide, N,N-dimethylaminoethylacrylamide, N-tert-
butylacrylamide, N-vinylformamide, N-vinylacetamide, acrylonitrile,
methacrylonitrile, or
any mixtures thereof.
15 Suitable monomers showing a hydrolysable carbonic acid ester function under
the
conditions of the application are selected from nonionic monomers, for example
water-
soluble or water-dispersible esters of acrylic acid or methacrylic acid, such
as
hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate (as a technical
grade
product, an isomer mixture), esters of acrylic acid and methacrylic acid which
possess,
20 as a side chain, polyethylene glycol, polypropylene glycol or copolymers of
ethylene
glycol and propylene glycol, and ethyl (meth)acrylate, methyl (meth)acrylate,
2-
ethylhexyl acrylate.
In addition, it is possible to use amino esters of acrylic or methacrylic
acid, since these
25 too are deprotonated very rapidly in cementitious systems (high pH) and
hence are
present in neutral form. Possible monomers of this type are dimethylaminoethyl
(meth)acrylate, tert-butylaminoethyl methacrylate or diethylaminoethyl
acrylate. Useful
crosslinkers include especially all representatives either hydrolysable or not
hydrolysable under the conditions of the application already specified in
connection
30 with process variant a), which can also be used in this case a) in the
proportions
specified there in each case.
In the case of variant d), the pure embodiment shall be understood to be that
in which
exclusively crosslinkers not hydrolysable under the conditions of the
application are
used.
Mixed embodiments:
Finally, the invention includes any desired combinations of the four process
variants a),
b), c) and d): in many cases, it is advisable to combine the different
variants (a+b+c+d;
CA 02703043 2010-05-13
31
a+b+c; a+b+d; b+c+d; a+c+d; a+b; a+c; a+d; b+d; c+d). One possibility is in
particular
the step of gel polymerization or inverse suspension polymerization. A further
aspect of
the present invention can therefore be considered to be that of an SAP which
has been
prepared with the aid of at least two process variants a), b), c) and d) and
preferably
employing gel polymerization and/or an inverse suspension polymerization. It
is easily
also possible for a crosslinker showing a hydrolysable carbonic acid ester
function
under the conditions of the appiication to be introduced into a monomer
solution
composed of an anionic monomer and a cationic, monomer that can release its
cationic
charge at a pH >7 by ester hydrolysis and/or deprotonation, in addition to the
crosslinker not hydrolysable under the conditions of the application. When
such a
polymer is used as a core polymer for the surface coating, the three variants
a), b) and
c) are implemented in the preparation of the inventive SAP.
Among all embodiments, variants a), b) and c), and the combination of variants
a), b)
and d), are preferred, since they need only one process step (gel
polymerization or
inverse suspension polymerization), while embodiments which make use of
variant c)
require three process steps (synthesis of the core polymer, synthesis of the
shell
polymer, surface coating) or lead to prolonged residence times in the reactor.
With all process variants described, it is possible to prepare superabsorbent
polymers
with anionic and/or cationic properties and a retarded swelling action, which
have
defined particle sizes. Since, in the context of the present invention, the
SAPs are
introduced into different pores and fissures of the underground formations, it
would be
disadvantageous in accordance with the invention only to use SAPs with a
specific
particle size. The present invention therefore also encompasses a further
process
variant in which the SAP has a particle size of 0.5 to 1000 pm, preferably of
1.0 to
200 pm and more preferably of 10 to 100 pm. The particle sizes mentioned can
be
varied with respect to one another and combined as desired.
A main aspect relates to the retarded swelling of the inventive SAPs, which
has already
been described in detail. In this connection, the present invention includes a
specific
process variant in which, 30 min after provision of the construction chemical
mixture
including the inventive SAPs, not more than 70%, preferably not more than 60%
and
more preferably not more than 50% of the maximum absorption capacity of the
superabsorbent polymer has been attained. In the context of the present
invention, this
maximum absorption capacity is determined in an aqueous salt solution which
contains
4.0 g of sodium hydroxide or 56.0 g of sodium chloride per litre of water.
Overall, it can be stated in summary that the main subject of the present
invention
CA 02703043 2010-05-13
32
consists in the specific use of superabsorbent polymers, which are defined by
specific
preparation processes and combinations thereof, and which feature, more
particularly,
a retarded swelling action with a commencement of swelling no earlier than
after
minutes. The swelling behaviour differs from the superabsorbent polymers known
to
5 date principally in that the liquid absorption, by virtue of the specific
structure of the
SAP, occurs with a time delay in the region of minutes. This is in contrast to
the known
applications in the hygiene sector, where a specific value is placed on the
fact that
(body) fluids are absorbed completely by the polymer within a very short time.
By virtue
of the retarded swelling and absorptive action of the inventive superabsorbent
polymers, it is thus possible to control the time of blocking of permeable
formations in
the exploration and exploitation of underground mineral oil and/or natural gas
deposits,
and also to adjust the amount of flooding medium required to the particuiar
specific
application.
The examples which follow illustrate the advantages of the present invention
without
restricting it thereto.
CA 02703043 2010-05-13
33
Examples
Abbreviations:
MADAMEQUAT = [2-(methacryloyloxy)ethyl]trimethylammonium chloride
TEPA = tetraethylenepentamine
DIMAPAQUAT = [3-(acryloylamino)propyl]trimethylammonium chloride
DEGDA = diethylene glycol diacrylate
Polymers with the following composition were synthesized:
Polymer 1(coating of an anionic superabsorbent polymer with a cationic shell
polymer)
A 2 I jacketed reactor was initially charged with 1000 g of cyclohexane. After
the
addition of 6 g of Span 60 protective colloid, 100 g of finely ground
acrylamide/acrylic
acid copolymer (Luquasorb AF 2 from BASF SE) were added and suspended. After
heating to 70 C, 250 g of a 10% shell polymer solution, which is a 1:1
copolymer of
acrylamide and MADAMEQUAT, were slowly added dropwise and the temperature was
increased to such an extent that the water added was removable by azeotropic
distillation. Once the azeotrope temperature had reached 72 C, the mixture was
cooled
below the boiling temperature. After the slow addition of a further 250 g of
shell
polymer solution, the mixture was heated again to boiling and water was
separated out
until the azeotrope temperature was 75 C.
After cooling, the mixture was filtered and washed with a little ethanol.
The shell polymer was prepared as follows:
A 10 I jacketed reactor was initially charged with 1.6 kg of water. Then 200.4
g of
acrylamide (50% solution in water) and 133.4 g of MADAMEQUAT were added, and
20% NaOH was used to establish a pH of 5. Subsequently, 38 g of water were
added
and the solution was purged with N2 for 30 min. During the purging, the
reaction
mixture was heated to 60 C. The polymerization is initiated by adding 380 ppm
of
TEPA and 2000 ppm of sodium peroxodisu(phate. The mixture was polymerized at
60 C for 2 h, cooled and transferred.
Polymer 2
A 2 I three-neck flask with stirrer and thermometer was initially charged with
99 g of
water and then 186.1 g of Na-AMPS (50% aqueous solution), 140.2 g of
DIMAPAQUAT, 216.9 g of acrylamide (50% aqueous solution), 13.8 g of DEGDA and
14.3 g of methylenebisacrylamide (2%) were added successively. After the
addition of
CA 02703043 2010-05-13
34
a further 89 g of water, adjustment to pH 5 with 20% H2SO4 and purging with N2
for 30
min, the mixture was cooled to 10 C.
The solution was then transferred into a plastic container with the dimensions
(wxdxh)
15 cm x 10 cm x 20 cm, and then 200 ppm of 2,2'-azobis(2-amidinopropane)
dihydrochloride, 250 ppm of sodium peroxodisulphate, 8 ppm of sodium
bisulphite,
20 ppm of tert-butyl hydroperoxide and 3 ppm of iron(II) sulphate heptahydrate
were
metered in successively. The polymerization was initiated by irradiating with
UV light,
(Cleo Performance 40 M.
After approx. 2 h, the resulting gel was taken from the plastic container and
cut into
cubes of edge length approx. 5 cm with scissors. Before the gel cubes were
comminuted by means of a conventional meat grinder, they were painted with the
separating agent coconut fatty acid diethanolamide.
The resulting gel granule was distributed uniformly on drying grids and dried
to
constant weight (approx. 3 h) in a forced-air drying cabinet at approx. 120 C.
Approx.
300 g of a white, hard granule were obtained, which were converted to a
pufveruient
state with the aid of a centrifugal mill. The mean particle diameter of the
polymer
powder was from 30 to 50 pm and the proportion of particles which do not pass
through
a screen of mesh size 63 pm was less than 2%.
The time-dependent swelling behaviour of these polymers was evaluated in a
synthetic
high-salinity deposit water from the north German plain. The results are
compiled in the
table which follows:
Water absorption Water absorption Water absorption Water absorption
after I h [g of after 6 h [g of after 24 h [g of after 48 h [g of
water/g of water/g of water/g of water/g of
polymer] polymer] polymer] polymer]
Polymer 1 2.2 2.4 2.5 18.7
Polymer 2 7.0 9.4 11.0 13.5
As can be seen from the examples, the appropriate structure of the
superabsorbents
allows the commencement of water absorption and of swelling to be controlled.
For
example, swelling commences at a relatively late stage in the case of polymer
1,
whereas polymer 2 has already reached about half of its water absorption
capacity
after 1 h.
